1. Field of the Invention
The present invention relates to a cleaning agent for a harmful gas containing a fluorine gas or a fluorine-compound gas (hereinafter referred simply to as "fluorine-compound gas") and to a process for cleaning the harmful gas by using the cleaning agent. More particularly, it pertains to a cleaning agent useful for removing a harmful fluorine-compound gas containing fluorine atoms such as hydrogen fluoride, fluorine, tungsten hexafluoride, silicon tetrafluoride and boron trifluoride that are used mainly in semiconductor manufacturing industries and then exhausted therefrom, and to a process for cleaning the above-mentioned harmful gas thus exhausted by the use of the aforementioned cleaning agent.
2. Description of Related Arts There has been a steady rise in the kinds and consumption of fluorine-compound gases such as hydrogen fluoride, fluorine, tungsten hexafluoride, silicon tetrafluoride and boron trifluoride in recent years as semiconductor industry and optoelectronics industry continue to develop.
These fluorine-compound gases are indispensable for the growth of the film of crystalline silicon, amorphous silicon or silicon oxide or as etching gas in the production industries of silicon semiconductors and compound semiconductors. These flourine-compound gases are used in semiconductor manufacturing industries or the like as such or after being diluted with helium, argon, nitrogen, hydrogen or the like, and thereafter are exhausted therefrom as such or after being diluted with a gas such as nitrogen or air, and accordingly, the concentration of the exhaust gas is not always constant.
Since each of these fluorine-compound gases exerts an undesirable influence on human bodies and the environment on account of its high toxicity, it is necessary to clean a harmful gas containing any of these fluorine-compound gases after being used in the semiconductor manufacturing process prior to the discharge in the atmosphere.
As the means for removing a fluorine-compound gas or other halogen gas or a halogen-compound gas (hereinafter referred simply to as "halogen-compound gas") that is contained in an exhaust gas, there have heretofore been used, as a wet process, a process in which a harmful halogen-compound gas is brought into contact with an aqueous solution of an alkali such as sodium hydroxide to absorb and decompose the gas in a scrubber, spray tower, rotary fine-bubble generating unit or the like (refer to Japanese Patent Application Laid-Open Nos. 204022/1986, 125827/1987, etc.) and, as a dry process, a process in which a harmful halogen-compound gas is passed through a packed column which is packed inside with a cleaning agent in solid form comprising, as an effective ingredient, an oxide, carbonate or the like of magnesium, sodium or potassium (refer to Japanese Patent Application Laid-Open Nos. 232844/1988, 68051/1985, etc.) or a cleaning agent in solid form comprising soda lime as an effective ingredient to clean the harmful gas.
However, the above-mentioned wet process involves in general the problems that difficulties arise in the post-treatment of the liquid used for removing a halogen-compound gas and the process not only requires complicated large equipment but also needs much expense in equipment and maintenance. On the other hand, the aforesaid dry process in which an adsorbent such as an oxide, carbonate or the like of magnesium, sodium or potassium is frequently used, suffers from the disadvantage of low removal capacity for fluorine-compound gas per unit volume of the adsorbent. There is also known a cleaning agent obtained by impregnating activated carbon with an aqueous solution of a zinc compound and an alkali metal compound. The cleaning agent just mentioned, however, involves the problems that the removal capacity for the harmful gas is insufficient as is the case with the above-mentioned adsorbent, thus failing to cope well with a concentrated fluorine-based gas when exhausted or with a large amount of gas to be treated. In addition, in the case of treating a highly reactive gas such a fluorine, the activated carbon-compound cleaning agent is involved in a possible danger that fire due to its flammability could break out.
In the case of the aforesaid cleaning agent in solid form by the use of soda lime, the cleaning capacity for the harmful gas is somewhat higher than that of any other cases, but the cleaning agent involves the problems that said agent is dried out, and when the exhauted fluorine-compound gas has a low concentration, the cleaning capacity is reduced to a great extent.
Under such circumstances, it has long been desired to realize a cleaning agent and cleaning process that have excellent cleaning capacity for a harmful gas containing various fluorine-compound gases in low concentrations and in a dry state which gas is exhausted from a semiconductor manufacturing industry or the like, and that are capable of removing said gas in high efficiency without any danger of causing fire at the time of cleaning.